1. Field of the Invention
The present invention relates to an energy storage device cell with a built-in configuration of a lithium ion capacitor and a lithium ion battery and a control method thereof.
2. Description of the Related Art
Energy storage device cells include electric double layer capacitors, lithium ion batteries, and lithium ion capacitors. An electric double layer capacitor (also simply called a capacitor, a super capacitor, an electrochemical capacitor, or the like, and a lutetium ion capacitor to be described later is also inclusively referred to as a capacitor), is provided with polarizable electrodes (a cathode and an anode) mutually facing across a separator, and makes use of capacitance of an electric double layer to be formed on the surface of the polarizable electrodes in an electrolyte solution.
A lithium ion battery has the advantage of being able to stably charge and store lithium in the carbon anode, and an oxide such as cobalt, nickel, or manganese is used for the cathode.
Moreover, as a new electric double layer capacitor, a lithium ion capacitor has been developed. The lithium ion capacitor is prepared by doping lithium ions in the anode of an electric double layer capacitor, and has a characteristic that the lower limit voltage cannot be lowered to 0V, although a higher upper limit voltage than that of the electric double layer capacitor can be obtained.
The electric double layer capacitor has no such an instantaneous power as in an aluminum electrolytic capacitor, but has an advantage of a large power density and thus being able to charge and discharge in a short time. On the other hand, the lithium ion battery is an energy storage device having an overwhelming energy density, that is, endurance among energy storage devices. If an energy storage device cell having both the instantaneous power of an electric double layer capacitor and the endurance of a lithium battery can be realized, this can be utilized for various applications, such as hybrid automobiles and various types of brake regeneration.
As an energy storage device cell with a built-in structure of an electric double layer capacitor and a lithium ion battery, a parallel connection of an electric double layer capacitor and a lithium ion battery and a mixture of a cathode active material (lithium cobalt oxide) of a lithium ion battery and a cathode material (activated carbon) of an electric double layer capacitor have been disclosed (see JP-A-2001-351688, page 3, FIG. 1, for example).
Also, with regard to the cathode active material of a lithium ion battery, lithium cobalt oxide is generally used, however, because it easily generates heat when short-circuited, and for the purpose of breaking away from a rare metal of cobalt to reduce costs, a method using, as a cathode active material, a mixture of olivine lithium iron phosphate (LiFePO4) and porous carbon and using, as an anode active material, a mixture of spinel lithium titanate (Li4Ti5O12) and porous carbon has been disclosed (see JP-A-2005-158719, page 4, FIG. 1, for example). It has been disclosed that, in a lithium ion battery thus constructed, a capacitance effect of the mixed porous carbon moderates a decline in voltage in a short time at the time of discharge.
However, in energy storage device cells with the structure where an electric double layer capacitor and a lithium ion battery are connected in parallel or with the structure where a cathode active material of a lithium ion battery and a cathode material of an electric double layer capacitor are mixed, there has been a problem such that, as charging and discharging are repeated, a phenomenon occurs such that, when anions (PF6−) are released from micropores of the cathode material (activated carbon) of the electric double layer capacitor, lithium ions from the lithium ion battery enter the micropores to oxidatively decompose the solvent and the anions (lithium ions) can no longer exit (pore blockage of activated carbon pores), so that charging and discharging characteristics decline. Particularly, when charging and discharging are repeated at a frequency of less than 10 second intervals or when charging and discharging voltage is raised, there has been a problem such that deterioration in electrical characteristics such as a decline in capacitance and a rise in internal resistance considerably accelerates. It has been discovered that, in either structure described above, opposing reactions to the electrolyte such that, at the time of charging, cations are released from the lithium ion battery, whereas anions are inserted in the electric double layer capacitor, and at the time of discharging, cations are inserted in the lithium ion battery, whereas anions are released from the electric double layer capacitor occur within the same plane or in proximity, and for this reason, deterioration in electrical characteristics is accelerated.
Moreover, although a structure where the anode of an electric double layer capacitor and the anode of a lithium ion battery are separated can also be considered, in such a structure, there has been a problem such that, when such a fast charge and discharge is performed as described above, electrical characteristics are easily deteriorated due to a problem such that the electrochemical potential of the anode of the electric double layer capacitor transiently drops excessively to cause reductive decomposition of the electrolyte solution, and a viscous coating resulting from a decomposition product of the electrolyte solution is produced on the surface of activated carbon particles or gas is generated.
Further, even when an energy storage device cell is constructed by connecting a lithium ion battery using, as a cathode active material, a mixture of olivine lithium iron phosphate (LiFePO4) and porous carbon and using, as an anode active material, a mixture of spinel lithium titanate (Li4Ti5O12) and porous carbon in parallel with an electric double layer capacitor, or by mixing a cathode active material (lithium cobalt oxide) and a cathode material (activated carbon) of an electric double layer capacitor, there has been a problem such that, opposing reactions to the electrolyte such that anions are released from the electric double layer capacitor, and at the time of discharging, cations are released from the lithium ion battery, whereas anions are absorbed in the electric double layer capacitor occur at proximate places, and for this reason, deterioration in electrical characteristics is accelerated.